J. Am. Chem. Soc. 1994,116, 6361-6367
6361
Chemical Shielding Tensor of a Carbenet Anthony J. Arduengo, III,*a David A. Dixon,*a Kristin K. Kumashiro,l Chengteb Lee,ll William P. Power,$**and Kurt W. Zilm'J Contributionfrom the DuPont Science and Engineering Laboratory, Experimental Station, P. 0. BOX 80328, Wilmington, Delaware 19880-0328, Department of Chemistry, Yale University, 225 Prospect Street, P. 0. Box 6666, New Haven, Connecticut 06511-81 18, and Cray Research, Inc., 655 East Lone Oak Drive, Eagan, Minnesota 55121 Received December 27, 1993.
Abstract: The first experimental determination of the chemical shielding tensor of a singlet carbene center, 1,3,4,5tetramethylimidazol-2-ylidene(l),is reported. Detailed ab initio theoretical calculations of the chemical shielding tensors are reported for local density functional theory (LDFT) and the Hartrce-Fock levels (HF) in the LORG and IGLO frameworks. The chemical shieldinganisotropy is quite large, which is characteristic of the lowest-energy-singlet carbene centers. The anisotropy at the carbene center in 1is -240 ppm. This is one of the largest anisotropiesobserved for carbon in a strictly organic framework. The calculations indicate that the orientation of the chemical shielding tensor of the carbene is such that the most shielded component is perpendicular to the molecular plane, with the intermediate component oriented approximately along the direction of the lone pair, and the least shielded component perpendicular to the other two. This orientation, as well as the relative size of the anisotropy, a g r a with calculations on the singlet carbenes 1, imidazol-Zylidene (2), and :CF2. The size of the anisotropy is predicted to be enhanced for the parent carbene :CH2.
Significant efforts have been expended to characterize and understand the electronic nature of carbenes.1 These highly reactive species possess a unique chemistry among carbon compounL,s5 however, this reactivity has precluded much detailed study of their structure by conventional experimental means, generally permitting only simpleinterpretation of reaction products or kinetics. Absorption spectra have provided details of the structure of many simple carbenes, and theoreticians have been particularly active in providingdescriptionsof the electronic and molecular structure based on ab initio molecular orbital calculations.6J While ESR spectroscopy has been successful in providing information about the electronic structure of several matrix-isolated triplet carbenes: the electronicstructure of ESRsilent singlet carbenes has yet to be fully investigated by spectroscopicmeans. We have recently reported the experimental and theoretical determination of the electron density in the stable singlet carbene 1,3,4,5-tetramethylimidazol-2-ylidene-d12.9 One of the most successful techniques for providing structural and electronic information about closed-shell species is solidstate NMR spectroscopy. Coupled with matrix-isolation techniques, solid-stateNMR has been used to identify various reactive 7 DuPont Contribution No.6793. t DuPont.
8 Yale University. I Cray Reseerch. A
Current address: Department of Chemistry, University of Waterloo,
Ontario, Canada N2L 3G1. Abstract published in Advance ACS Abstrucrs, June 1, 1994. (1) Regitz, M. Angew. Chem, Int. Ed. Engl. 1991,30,674.
(2) Baron, W. J.; Bertoniere, N. R.; Decamp, M. R.; Griffii, G. W.; Hendrick, M. E.;Jones, M., Jr.; Levin, R. H.; Moss, R. A.; Sohn, M. B. Carbenes. In Curbenes;Jones, M., Jr., Moss, R. A., E&.; John Wilev & Sons: New York, 1973. (3) Clw, G. L.; Gaspar, P. P.; Hammond, 0.S.;Hartzler, H. D.; Mackay, C.; Seyferth, D.; Trozzolo, A. M.; Wasserman, E. Carbenes. In Cwbenes; Moss, R. A., Jones, M., Jr., Eds.;John Wiley & Sons: New York, 1975. (4) K i m , W. Curbene Chemistry: - - Academic Press. Inc.: New York, 1971; VOI. 1. (5) Hine, J. Diuulenr Carbon; The Ronald Press Co.: New York, 1964. (6) Harrison, J. F. Acc. Chem. Res. 1974, 7, 378. (7) Schaefer, H. F., 111. Science 1986, 231, 1100 and references therein. (8) Perutz, R. N. Chem. Rm. 1985,85, 77. (9) Arduengo, A. J., 111; Dias, H. V. R.; Dixon, D. A.; Harlow, R. L.; Klooster, W. T.; Koetzle, T. F. J. Am. Chem. Soc., in pres.
intermediates in organic chemistry, through interpretation of isotropic chemical shiftslOJ1 and identification of the number of protons bonded to the reactive species.12 Characterization of the anisotropy or threedimensional character of the chemical shielding provides an even more powerful probe of the electronic environment around the nucleus of interest, allowing the orientation of particular shielding or deshielding influences to be correlated with the local molecular structure. Such an approach is especiallyinformative when coupled with theoreticalpredictions of the chemical shielding tensor. Our preparation of a series of stable crystalline carbenes13J4 has permitted us to investigate these normally transient species using solid-stateNMR techniques in order to further characterize their electronic and structural features. In this report, we concentrate on experimental results for 1,3,4,5-tetramethylimidazol-Zylidene (1). These results are then compared toab initio molecular orbital and density functional calculations on 1 and the parent imidazol-Zylidene (2) as well as the prototypical carbenes :CH2 (3) and :CF2 (4). Comparisons are also made with carbenium ions 1-H+ and 2-H+ that are derived from protonation of 1 and 2 respectively.
Experimental Seetion Samplesof 1,prepared asdescribed previously~J4werescaledin5-1" 0.d. glass tubes under helium. The precursor salt, 1,3,4,5-tetramethylimidazolium chloridc9J4 (ISH+Cl-), was packed under argon into a zirconia rotor. Carbon-13 solid-state NMR spectra were obtained with cross-polarization (CP), high-power proton decoupling, and magic-angle spinning (MAS) on both a custom-built 7.05-T NMR spectrometer operating at 75 MHz for 13Canda custom-built 2.35 TNMRspectrometer operating at 25 MHz for 13C. Proton */2 pulse widths were 5 ps, corresponding to a proton decoupling field of 50 W z . A CP time of 3 ms was used for all samplts at both fields. Acquisition times of 51 m (10) Zilm, K. W.; Memll, R. A.; Greenberg, M. M.; Bemn, J. A. J. Am.
Chem. Soc. 1987,109, 1567.
(1 1) Greenberg, M. M.; Blackstock, S.C.; Bemn, J. A.; Merrill, R. A.; Duchamp, J. C.; Zilm, K.W.J. Am. Chem. Soc. 1991,113,2318. (12) Zilm, K. W.; Merrill, R. A.; Webb, G. G.;Greenberg, M. M.; Berson, J.A. J. Am. Chem. Soc. 1989,111, 1533. (13) Arduengo, A. J.; Harlow, R. L.;Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (14) Arduengo, A. J., III; Dias, H. V. R.; Harlow, R. L.; Kline, M. J . Am. Chem. Soc. 1992, 114, 5530.
OOO2-7S63/94/1516-6361$04.50/0 Q 1994 American Chemical Society
Arduengo et al.
6362 J. Am. Chem. SOC.,Vol. 116, No. 14, 1994
C-LjHS
H
rx3
2
1
4do.O
35b.O 3Ob.O
2G.O 2Ob.O
lh.0 100.0
4.0
Of0
PPM Figure 1. Carbon-13 CP/MAS spectrum of 1, with spinning of 1.8 kHz at 7.OSTand 293 K.Theisotropicshiftsofeachcarbonhavebeenindicated on the spectrum. The spinning sidebands corresponding to the carbene site have been marked by a vertical line above the spectrum.
2.H’
*>c
H
3
:
F
:
F>C
4
were used, during which 2048 complex data points were collected. Magicangle spinning rates up to 2.5 kHz in custom-built double-resonance MAS probes at each field strength were used to assign the isotropic peaks of 1. A Doty Scientific double-resonance MAS probe for the 7.05-T spectrometer was used to spin samples of the precursor at rates up to 5 kHz. Carbon-13 CP/MAS spectra of 1 at 25 MHz were collected at both room temperature (293 K) and 90 K, spectra at 75 MHz were collected at room temperature only. All spectra are referenced with respect to tetramethylsilane (TMS); this was accomplished using external samples of adamantane and hexamethylbcnzene.
Theoretical Calculations The density functional theory calculations~~19 were done with the program DGauss,20-22 which employs Gaussian basis sets on a Cray YMP computer. The basis sets for C, N, and F are triple-{in thevalence space augmented with a set of polarization functions with the form (71 11/41 1/1),23The auxiliary fitting basis set for the electron density and the exchange-correlation potential has the form [8/4/4]. For H, apolarized triple-{valence basis set was used with the form (3 11/ 1) together with an auxiliary fitting basis set of the form [4/ 11. This basis set is labeled TZVP. The calculations were done at the local density functional level with the local potential of Vosko, Wilk, and N u ~ a i rFor . ~ ~2, 3, 4,l-H+and 2.H+, the geometries were optimized with this basis set at the self-consistent gradient-corrected (nonlocal) level with the nonlocal exchange potential of BeckezSz7 together with the nonlocal correlation functional of Perdew28 (BP). Following our (15) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (16) Salahub,D. R.InAb InitioMethodsinQuantumMethodsinQuantum Chemistry-II;Lawlwy, K. P., Ed.; J. Wiley & Sons: New York, 1987; p 447.
(17) Wimmer, E.; Freeman, A. J.; Fu, C.-L.; Cao, P.-L.; Chou, S.-H.; Delley,B. Supercomputer Research in Chemistry and Chemical Engineering; ACS Symposium Series 353, American Chemical Society: Washington, DC, 1987. (18) Jones, R. 0.; Gunnarsson, 0. Rev. Mod.Phys. 1989, 61,689. (19) Zeigler, T. Chem. Rev. 1991, 91, 651. (20) Andzelm, J. W.; Wimmer, E.; Salahub, D. R. The Challenge of d and fElectrons: Theory and Computation;ACS SymposiumSeries 394,American Chemical Society: Washington, DC, 1989. (21) Andzelm, J. W. Density Functional Methods in Chemistry; Springer-Verlag: New York, 1991. (22)Andzelm, J. W.; Wimmer, E. J. Chem. Phys. 1992, 96, 1280. (23) Godbout, N.; Salahub, D. R.; Andzelm, J. W.; Wimmer, E. Can. J . Chem. 1992, 70, 560. (24) Vosko, S. J.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (25) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (26) Becke, A. D. The Challenge of d and f Electrons: Theory and Computation; ACS Symposium Series 394, American Chemical Society: Washington, DC, 1989. (27) Becke, A. D. Int. J. Quantum Chem., Quantum. Chem. Symp. 1989, 23, 599.
work on analysis of the electron density of the carbene 1, we used its experimental geometry from a neutron diffraction study? Calculations of the NMR shifts were done at the LDFT level in two ways. The calculations handle the gauge invariance problem in terms of the localized orbital/local origin (LORG)*9 and individual gauge localized orbital (IGLO)3O-32 methods following the recent derivation of Lee33 based on the work of Salahub and co-worken” and Handy and co-worke1s.3~Calculations were done with the TZVP basis set described above. The calculations at the Hartree-Fock level were done with the program CADPAC.36Two basis sets were used. The first is of polarized double-{ quality (DZP) .37 For all molecules except for 1and l.H+, calculations were also done with a triple-{ basis set augmented by two sets of polarization functions (TZ2P).3* For 1 and ISH+,a TZ2P calculation cannot be done in CADPAC (369 basis functions). Because our focus is on the ring atoms and primarily on CZ,calculations on 1 and 1*H+were done with a modified TZ2P basis set denoted as ‘TZ2P“. The “TZ2P” basis set had a TZZP basis set for the ring atoms, a DZP basis set for the methyl carbons, and DZ for H except for the unique hydrogen at C2 of l.H+, for which a DZP basis set was used.39.u Chemical shieldingcalculationsweredone at thecoupled perturbed HartreeFock level (CPHF) and at the IGLO and LORG levels.
Results and Discussion A 13CCP/MAS spectrum of the carbene 1is shown in Figure 1. The isotropic shift of the carbene carbon in the solid, 209.6 ppm, is very similar to that observed in solution for this compound, 21 3.7 ppm.14Theother isotropicpeaks in the solid-statespectrum closely match the solution data, appearing at 124.7 ppm (C,,,,), 34.6 ppm (N-CH,), and 9.5 ppm (C-CH3). The solid-state and solution isotropic shifts for the carbons in 1 and l*H+are given in Table 1along with the predicted values from various theoretical (28) Perdew, J. P. Phys. Rev. E 1986, 33,8822. (29) Hansen, A. E.; Bouman, T. D.J. Chem. Phys. 1985,82, 5035. (30) Kutzelnigg, W. Isr. J . Chem. 1980, 19, 193. (31) Schindler, M.; Kutzelnigg, W. J. Chem. Phys. 1982, 76, 1919. (32) Kutzelnigg,W.; Fleischer,U.; Schindler, M. The IGLO-Method: A b Initio Calculation and Interpretation of NMR Chemical Shifts and Magnetic Susceptibilities. In N M R Basic Principles and Progress; Diehl, P., FlucL E., Gunther, H., Kosfeld, R., Seelig, J., Eds.;Springer: Berlin, 1991; p 165. (33) Lee, C.; Fitzgerald, G.; Dixon, D. A.; Kleier, D. A. J. Phys. Chem., submitted. (34) Malkin. V. G.; Malkina, 0.L.; Salahub, D. R. Chem. Phys. Lctr. 1993. 204. 80.87. (35) Smith; C. M.; Amos, R. D.; Handy, N. C. Mol. Phys. 1992,77,381. (36)Amos, R. D.; Rice, J. E. CADPAC. The Cambridge Analytic Derivatives Packane. Version 5.2. 1987. (37) Dunning,-T.’H., Jr. J. Chem. Phys. 1970, 53, 2823. @(H) = 1.0; fd(C) = 0.8; fd(N) = 0.8; N(F) = 1.2. (38) Dunning, T. H., Jr. J. Chem. Phys. 1971,55, 716. @(H) = 1.5,0.5; fd(C) = 1.2, 0.4; fd(N) = 1.35, 0.45; fd(F) 2.0, 0.6667. (39) The use of ‘locally dense” basis set schemes has been previously described for NMR shielding calculations, scc ref 40 and the following: Chesnut, D. B.; Moore, K. D. J. Comput. Chem. 1989, 1989, 648. (40) Chesnut, D. B.;Rusiloski, B. E.; Moore, K. D.; Egolf, D. A. J. Comput. Chem. 1993,14, 1364.
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Chemical Shielding Tensor of a Carbene
J. Am. Chem. Soc.. Vol. 116, No. 14, 1994 6363
Tabk 1. Experimental and Calculated' Chemical Shiftsb(ppm) in 1 and l.H+ experimental
solutionc
SS CP/MAS
213.7 123.1 35.2 9.0 -198.5 3.48 2.01
209.6 124.7 34.6 9.5
135.0 126.8 32.7 7.3 -206.6 3.72 2.20 9.28
137.0 129.7.127.6 35.3, 34.0 9.9,7.0
HF/DZP IGLO LORG 1 179.1 105.9 16.5 -6.0 -230.9 -11.3 -12.9
HFf"TZ2P''d IGLO LORG
LDFT/TZVP IGLO LORG
0.4
226.0 112.6 26.0 2.5 -216.5 -8.3 -9.9
236.6 122.5 30.7 7.8 -207.5 -1.5 -2.5
188.8 101.1 28.6 3.9 -193.9 -1 .o -1.9
196.8 106.2 33.6 7.2 -195.3 6.6 4.6
139.7 135.3 32.3 8.4 -208.8 2.1 1 .o 7.2
136.6 123.4 29.7 4.2 -211.4 -7.4 -9.1 0.6
143.0 133.2 34.4 9.2 -201.4 -0.6 -1.8 7.3
121.2 123.2 33.5 7.7 -198.4 -0.8 -2.7 -0.1
127.0 127.3 38.6 10.6 -195.2 7.1 3.9 7.2
203.9 124.4 28.0 6.9 -215.0
1.1
l*H+ 126.7 116.7 20.4 -4.4 -226.0 -10.5 -12.2 -9.0
-
a Calculated absolute isotropic shielding8 have been collvcrtcd to c h e d a l shifts by the relationship drrc = 186.4 - U L O ; 61% ~ ~= ~ -131.8 ULO;~ 61, = 30.6 ULO + 0.2 (Ab(CH4- TMS)).4sb References are (CH3)fii for iH and and 5 N NH4+iVO3-for W .e Solution data for 1were recorded from a THF-dn solution, and solution data for l.H+ were recorded from DMSO-& The 'TZ2P" basis set used for 1and l.H+ is TZ2P for Q, Cyq, and Nl(3), DZP for the methyl carbons, and DZ for H.
-
treatments. Note that only single peaks occur for each of the chemically distinct carbon sites in 1,confirmingthat the molecule has a high degree of symmetry in the C 2 / c crystal lattice, with the molecule sitting on a 2-fold axis, as previously determined.14 Nonequivalence of the two halves of the molecule would be expected to lead to a doubling of the methyl and alkene peaks. Such doubling is evident in the 13C CP/MAS spectrum of the precursor salt, 1,3,4,5-tetramethylimidazoliumchloride (&H+ C1-). In the solid-state spectrum of l.H+ C1-, all peaks other than the methine carbon, C2, are doubled (Table 1). These doubled resonances reflect the asymmetric environmentthat l.H+ occupies in a general position of a PZl/ccrystal lattice," but the resonances are close to their corresponding solution values. The only chemical shift that has changed appreciably between l*H+and 1 is that of C2, the carbene center; the other carbon (and nitrogen) nuclear environments appear to be relatively unaffected by the loss of the ring proton. The origin of this chemical shift change at C2 is evident from the chemical shielding tensor at this center, as discussed below. The spinning sidebands of the carbene peak in 1, occurring at integer multiples of the spin rate to either side of the isotropic peak, cover a spectral width of approximately300ppm, indicating a largechemical shieldinganisotropy for this site, with appreciable sideband intensity from near 90 ppm up to 360 ppm. By using Herzfeld-Berger analysis of the spinning sideband intensities,& it is possible to reconstruct the powder pattern and determine the three principal components of the carbon chemical shielding tensor. For this reconstruction, it is further necessary to assume that dipolar contributions from the neighboring 14N nuclei are negligible. Support for this assumption is found in the absence of any asymmetric splitting of the carbene signals due to coupling with the quadrupolar nitrogen nuclei at 7.05 T.47The small magnitude of the C-N dipolar coupling constant, calculated to (41) Arduengo, A. J.,III;Harlow,R. L.;Dias,H. V.R.unpublshed resuits. (42) Jameson, A. K.; Jameson, C. J. Chem. Phys. Lett. 1987,134,461. (43) A more reccnt eatimate of ~ ~ ~ ( K)3 00.60 f 0.9 ppm os the value of 1.0 f 1.2 ppm used in ref 42 would lead to slightly different estimates for LVX from our calculated shieldings;see: Raynes, W. T.; McVay, R.; Wright, S. J. J. Chcm. Soc., Faraday Trans. 2 1989,85,759. (44) Jameson, C. T.; Jameson, A. K.; Oppusunggu, D.; Wille, S.; Burrell, P. M.;Mason,J. J. Chem. Phys. 1981, 74, 81. (45) Raynea, W. T.Theoreticaland Physical Aspectsof NuclearShielding. In Nuclear Magnetic Resonance;Abraham, R. J., Ed.; The Chemical Society, Burlington House: London, 1978. (46) Herzfeld, J.; Berger, A. E. J. Chcm. Phys. 1980, 73, 6021. (47) Hexem, J. G.; Frey, M. H.; Opella, S. J. J. Chem. Phys. 1982, 77, 3841.
-
be 860Hz using the X-ray crystallographicallydetermined bond lengthof 136.3pm,141iitstheerrorintroducedbyt~assumptim to agreement with the experimental values, as are the other components. W e should point out that the calculated results are T h e calculated N M R shifts for all of the atoms in 1 are shown for an isolated gas-phase molecule a t 0 K with no vibrational in Table I. Those for the protons were obtained by averaging averaging nor medium effects, as compared to the experimental the three protonsat each methyl. Theagreement withexperiment values, which were obtained in the solid a t room temperature. is better with the larger basis set and the LORG treatment of the T h e calculated shifts and tensors for the parent imidazol-2gauge problem a t the H F level. At the LDFT level, the LORG ylidene, 2, show the need for including the methyl groups for method seems to give values closer to experiment than does the comparison toexperiment, even though the substituent variation IGLO method. Both the HFl'TZ2P"lLORGand DFT/TZVP/ is one atom removed from the carbene center. Although the LORG calculationsgive reasonablepredictions ofthe NMRshifts trends in the tensor values are the same in 2 as in 1. the values relative to TMS. T h e LDFT N M R shifts for the ring carbons are not. The value of all is smaller in 2 than in 1. and the value are too low by - I 7 ppm independent of whether the carbon is of nl1 is larger in 2 than in 1; only the value of uzzis comparable in the double bond or is the carbene center. Although the proton between the two compounds. shifts are in the correct order, the values are too low and the More information about the carbene carbon N M R shift and splitting is too small at the H F level, whereas the opposite is true the shielding tensors is available from examination of the for the LDFT values. theoretical results on the model systems IAl :CH*, :CFz. and 2. T h e results are given in Table 2 a t the HF level and in Table 3 One feature of the carbene shielding tensor that is evident a t the L D F T level. Although. the largest error is expected for without ambiguity from thecalculations is itsorientation,depicted :CHl a t the H F level as this carbene has the largest coupling of in Figure 2. placing the most shielded component (a,,) perpenthe lone pair with the vacant p. orbital. the results clearly show dicular to the plane of the molecule, the intermediate component that an extremely low field shift is predicted for the carbene due ( a z l )along the direction of the carbene lone pair, and the least largely to the very low value for all. T h e value of the chemical shielded component (all) perpendicular to the other two within shielding is a factor of a t least 6 larger than the usual range given the plane of the molecule. of 1JCshifts. The qualitative trend given by the H F shifts for These tensor orientations are completely analogous to tensors :CHz is also found a t the L D F T level. If fluorine is substituted determined for carbon nuclei with a hydrogen in place of the for hydrogen in :CHz. the ground state becomes a singlet and the lone pair in aromatic environmentssJ and for two-coordinate singlet-triplet splitting becomes large. Difluorocarbene is now nitrogenJe56 and phosphorus~'-J9nuclei possessing a lone pair. morelike 1 (or 2),and the value of^,^ becomes much larger (less T h e calculated H F N M R tensor elements (Table 2) show an negative) than in ' A , :CHz. Thevalueof uzzalsoincreases, while obvious basis set dependence as well as a dependence on the aJldecreases. T h e extreme change in a11 is a result of changes treatment of thegauge problem. Thebest gauge treatment is the in the paramagnetic (deshielding) component of the tensor due LORG method. with the C P H F method providing the worst to the much lower energy of the n m* transition in IAl :CH2 ,521 Bccnurc the orientation or the chemical shielding tcnsor components than :CF2.*Thischangein the paramagneticcontribution to atI IS critical I J the dwurrionr of this work and the convcnuonaI a w m m c n t r dominates the changes in the diamagnetic component (uide infra). would allow o,, and oIIto vary with respect to the molecular a i s . & have T h e chemical shielding tensor we have characterized for the adapted.farihepurpoxJofthiraniclc.a convention that allowsthcdsignationr of the principal componcntr of the chemical shielding lcnsors to remain carbene site in 1 represents one of the most anisotropic tensors approrimatcly parallel to the assignments made a t thccarbcne e ~ n t einr I and reported forcarbon inastrictlyorganicframework.61Thenegative 2. Thus as illustrated in Figure 2, rtl at C2 in the cations l.H+ and 2.Hf value f o r a , , in 1suggestsacontinuingdominanceofthecarbeneremains along the C-H bond. with oIIperpendicular to the molecular plane and mII orthogonal to the other two wmponcnts (i.e., parallel to the C,-C, like resonance structure A over the fully *-bonded ylidicresonance bond). At the C4,jjcenter in I and l.H+, 022 is the dsignation ofthc primary structureB. the value of^,^ in 1, whichissomewhatlarger (less chemical shielding tensor eomponent that is most nearly parallel to 022 at C2 negative) than all in :CF2, is in accord with expcctations of a and o,, io thedcrignationofthcprimaryehcmicalrhiEldingtcnrarwmponcnt that is most nearly parallel to a,a t C2, while (111 remains perpendicular to dominant paramagnetic contribution to this tensor element.6zA the molecular plane. The exact aricntation ofthc primary chemical shielding maximally *-bonded model is provided by the CZcenter in I.H+. tcnoorcom~ncntratCI15, . ~is.rotated by an angleS(Figurc 2) from the principal where theshielding tensor appears typical for *-bonded aromatic molecular Bxcs. centers." In fact the shielding tensor a t C l of I.H+ (Table 4) is (53) Mehring. M. High Resolution NMR in Solids; Springer-Verlag: Berlin. 1983. (60) The .Al 'B, trmstt!on tn CHI IS - I I eV. ru Rauuhlichcr C (54) Warylirhen. R. E.: Power. W. P.; Penner. G. H.; Curtis, R. D. Con. W Chrm PhJr l z l l 1980.74.213 Hcrzkrg.G, Johns. J W C Pror R J. Chcm. 1989.67. 1219. Sor ILondonlSer AI966. 195 . 101 Shavitt I Trrrohrdron1985 41 I S I I (55) Wasylishen. R. E.; Penner. G. H.: Power, W. P.; Curtis. R.D. J . Am. The 'Al 'BItransition in :CF2 is -4.3 cV; ICC: Comes. F. J.; Ramray. 1. Chem. Soc. 1989. 111,6082. A.J. Mol. Specrrosc. 1985.113.495. King. D.S.;Schcnck.P. K.;Stcphcnron. (56) Curtis. R . D.;Penncr.G. H.; Power. W. P.; Wasylirhcn. R. E. J. Phys. J. C. J. Mol. Specrrosc. 1979. 78. I . C h m . 1990. 94. 4000. (61) Duncan. T. M. A Compilorion of Chrmieol Shin Anirolropirs; (57) Zilm, K. W.: Webb. G.G.;Cowley. A. H.; Pakulrki. M.;Orcndt. A. Farragut Press: Chicago. 1990. 1. Am. Chem. Soc. 1988. 110. 2032. (62) A gas-phase UV spectrum of the perdeuteri-analog of I (taken at (58) Duchamp. J. C.; Pakulski. M.; Covlcy. A. H.;Zilm. K. W. J . Am. room temperature under autogenous pressure) shows a strong absorption st Chcm. SOC.1990. 112.6803, 236nmandawvcakerrhoulderat -270 nm.Thislatterabsarption i o p i b l y (59) Curtis. R. D.;Royan. 8. W.; Warylirhen. R. E.; Lumrdcn, M. D.; the n - I * transition (4.6 eV). Burford. N. Inorg. Chrm. 1992. 31. 3386.
._
/
-
~~
-
~~~
~~~
I
~~
Arduengo et al.
6366 J. Am. Chem. SOC.,Vol. 116, No. 14, 1994
Table 4. Summarv ComDarison of Exmrimental and Calculated Absolute Shieldinn Tensors for Ring Carbons in 1 and l.H+ 0.b
a Shielding
atom
method
UiUl
Q11
CZ C4W CZ CY3 CZ CY5) CZ C40) CZ C4U) CZ CUSl
expt SS CPIMAS expt SS CPIMAS expt SS CP/MAS expt SS CPIMAS HFI'TZ2P"ILORG HFI"TZ2P"ILORG HFI"TZ2P"ILORG HFI'TZ2P"ILORG LDFTITZVPILORG LDFT/TZVP/LORG LDFTITZVPILORG LDFT/TZVP/LORG
-23.2 61.7 49.4 56.7 -50.2 63.9 43.4 53.2 -10.4 80.2 59.4 59.1
-184 (20) 67 (19) 45 (11)
the Chemical Shielding Tensors at the Carbene Centers in Various Carbenes ab
1
ISH+
55.0 -174.0 68.5 51.8 39.0
3.2 -19.2 -24.2 -28.9 14.0 14.7 9.6 -2.2
124.1 137.2 120.7 133.5 128.9 157.5 116.7 140.5
tensors are absolute in ppm. b Numbers in parentheses represent error in the last figures.
Table 5. Paramagnetic ( u 3 and Diamagnetic ( u a Contributions to
cmpd CHz CFz
-271.8 73.6 33.7
ui~l -1807.2 -130.5 -10.4 59.4
411e1 -5431.61276.4 -71 1.11264.3 -441.0/267.0 -264.51316.3
4/42 -732.31270.9 -333.21348.9 405.91419.9 413.71423.5
&
